Optical fibers and optical fiber devices with total dispersion greater than material dispersion

ABSTRACT

Disclosed are optical fiber devices incorporating optical fibers with total dispersion greater than material dispersion, and with preferred dispersion values less than +50 ps/nm-km. The desired dispersion values are obtained when light resides substantially in a single higher order mode (HOM) of the fiber, typically the LP 02  mode. The optical fibers also preferably have substantial separation between the effective indices of the HOM and any other mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/367,495, filed on Mar. 4, 2006 now abandoned, and having the title“Optical Fibers and Optical Fiber Devices with Total Dispersion Greaterthan Material Dispersion,” which is incorporated by reference herein asif expressly set forth herein in its entirety.

FIELD OF THE INVENTION

The invention relates to optical fibers devices wherein the totaldispersion of the device is greater than the material dispersion.

BACKGROUND OF THE INVENTION

An optical fiber can guide light with multiple spatial patterns, each ofwhich is uniquely designated as a transverse mode of the fiber(hereafter, called mode, for brevity). The dispersive characteristics ofan optical signal in a fiber depend on the mode in which it istravelling. Thus, each mode may be characterised with a dispersion valuespecific to it. The dispersion of a mode is roughly equal to the sum ofthe material dispersion (D_(m)) and waveguide dispersion (D_(w)). Thematerial dispersion is the dispersion of the material in which theoptical signal resides—that is, the material of which the fiber is made(most commonly, Silica with trace amounts of Germanium, Phosphorus,Fluorine and other dopants). The waveguide dispersion is due to therefractive index profile that defines a fiber waveguide. Hence, thedispersion of a mode (D_(total)=D_(m)+D_(w)) can be designed by suitablyaltering the refractive index profile of the fiber (which modifiesD_(w)). As will be explained below, for most optical fiber designs, thewaveguide dispersion D_(w), is negative. Hence, while the refractiveindex profile of a fiber can designed to obtain extremely large,negative values of D_(w), and hence the fiber dispersion D_(total), ofvarying negative magnitudes can be achieved, most fibers are bounded bythe material dispersion, in maximum achievable dispersion. Silica, evenwith a variety of dopants, has Dm>0 for wavelengths greater than roughly1300 nm, and has D_(m)<0 for wavelengths below 1300 nm. Hence, mostoptical fibers can achieve positive or negative dispersion (D_(total))for wavelengths greater than 1300 nm, but possess only D_(total)<0 forwavelengths below 1300 nm.

The optical response of an optical pulse in a fiber depends criticallyon the dispersion it experiences. This is true for both linear effectssuch as pulse spreading, and nonlinear effects such as pulse distortionand soliton formation. Hence, the dispersion of a fiber plays a key rolein designing fiber-based devices. Whereas optical fiber communicationssystems typically operate at 1300 nm or between 1500 and 1650 nm, manyother important optical systems operate at lower wavelengths. Apreferred wavelength of operation for fiber lasers is at 1060 nm. Theubiquitous titanium-doped sapphire laser, which is used in severalpump-probe experiments as well as in biomedical imaging or therapy,typically operates in wavelength range of 700 nm to 1000 nm. Finally,all wavelengths visible to the human eye, and hence wavelengths at whichseveral commercial gadgets such as the laser pointer work, spans therange of 400 to 700 nm. Common to all these applications is a wavelengthof operation below 1300 nm; where the silica fiber offers only negativedispersion. Fibers that have positive or zero dispersion in thesewavelength ranges would enable propagation of solitons and generatebroadband supercontinua, of interest to biomedical imaging systems, forinstance. For many of these systems, positive but low dispersion fibersare required at these wavelengths. Hence, there is a need for opticalfibers that can provide stable propagation for optical pulses ofwavelength less than 1300 nm, whose dispersion is positive and can beadjusted by suitably designing the refractive index profile. Thisrequires a fiber whose waveguide dispersion D_(w) can be designed to begreater than zero in any desired wavelength range.

Most optical fibers are single-moded, which means that they support onlythe lowest order, fundamental mode, also designated as the LP₀₁ mode.The two numerals in the subscript refer to the number of intensityminimas (zeroes) the spatial light pattern has, in the azimuthal (1^(st)subscript) and radial (2^(nd) subscript) directions, respectively. Asmentioned earlier, the LP₀₁, in standard silica fibers where therefractive index profile is defined by various dopants to silica, canachieve only D_(w)<0. Thus, the entire class of these fibers can have amaximum D_(total)=D_(m), the material dispersion of silica. SinceD_(m)<0 for wavelengths<1300 nm, it is not possible to achieveD_(total)>0 in this wavelength range.

Fibers that contain air holes that extend longitudinally along the axisof the fiber (called air-silica fibers, hereafter)-possess interestingproperties, as described by J. C. Knight and coworkers in volume 12,page 807 of the July 2000 issue of the IEEE Photonics TechnologyLetters, entitled “Anomalous Dispersion in Photonic Crystal Fiber.” Theydemonstrate that air-silica fibers can achieve large positive dispersionin any wavelength range. However, the dispersion of air-silica fibers isclosely tied to their modal areas, and it is not possible to achievehigh dispersion as well as large effective modal areas—hence, thisdesign space would be of limited use in systems requiring high positivedispersion but also low nonlinearities. In addition, these fibers areknown to have high birefringence and loss, both of which diminish theirutility in practical systems. Moreover, an all-solid fiber made byconventional technology will always be cheaper than fibers that requiremanual assembly of the fiber preforms (as is the case with air-silicafibers). These fibers also have termination problems—splices to otherfibers lead to loss, changes in optical properties, and cannot be madereliably.

Lysiansky, Rosenblit and Wei disclosed an alternative technique toobtain D_(w)>0. In U.S. Pat. No. 6,724,964, they disclosed exemplaryrefractive index profiles of a solid (i.e. not air-silica) fiber thatsupports higher order modes (HOM) in addition to the LP₀₁ mode, wherethe waveguide design yields dispersion greater than +50 ps/nm-km for theLP₀₂ or LP₀₃ mode. However, these designs do not enable achieving zeroor low positive dispersion values in the wavelength range<1300 nm, andhence cannot be utilized for applications such as soliton compressionand supercontinuum generation, typically exploited with lasers in thewavelength range of 700-900 nm. In addition, these HOM fibers sufferfrom a severe drawback common to most fibers that support more than asingle mode. While it is desired to have light residing substantially inthe desired HOM, the presence of other modes makes such designssusceptible to mode coupling, by which process light can either be lostor can cause deleterious interference-noise problems. Such mode couplingincreases as the difference in effective index (n_(eff)) between thedesired modes and any other mode, decreases. The design space disclosedby the above authors leads to identical n_(eff) for the LP₀₂ and LP₁₁modes, at the operation wavelengths. Hence these designs are especiallysusceptible to both interference noise and loss.

Hence, there exists the need for a fiber that can be manufactured byconventional fabrication techniques, whose refractive index profile issuch that it yields not only positive dispersion of any magnitude in anywavelength range, but also ensures that the modal spacings in the fiberare such that the fiber is not susceptible to mode coupling.

SUMMARY OF INVENTION

The present invention is directed to optical fiber devices incorporatingoptical fibers with refractive index profiles that yield D_(total)>D_(m)in any wavelength range such that D_(total)<+50 ps/nm-km, as would bedesired in a variety of fiber devices exploiting optical nonlinearities.The aforementioned refractive index profile yields the given dispersionvalues when light resides substantially in a single higher order mode(HOM) of the fiber. Typically, this HOM would be the LP₀₂ mode of thefiber, but those skilled in the art will realize that such designs canbe extended to other HOMs, such as the LP₁₁, or the LP₀₃ modes. We showexemplary profiles that provide small positive D_(total) (<+50 ps/nm-km)in the wavelength range of 820-900 nm, as well as 1.040-1160 nm, sincethese wavelength ranges are especially of interest for nonlinearfiber-optical devices, and it is in these wavelength ranges thatconventional silica fiber has D_(total)<0, motivating the need foralternatives.

Also disclosed here is a refractive index profile that yieldsD_(total)>D_(m) in any wavelength range, with no constraints on themagnitude of dispersion D_(total), but that simultaneously yieldsstable, mode-coupling-free propagation of the signal. To achieve thelatter characteristic, the fiber designs are constrained to those thatadditionally achieve a difference in n_(eff) (designated as Δn_(eff)hereafter) between the desired HOM and any other mode, of an absolutevalue greater than 10⁻⁴. Fibers, as those whose designs are disclosedhere, that achieve D_(total)>0 and absolute value of Δn_(eff)>10⁻⁴ willenable a variety of fiber devices in the wavelength ranges<1300 nm, forwhich only bulk-optic devices exist currently.

The present invention also relates to an apparatus for obtaining adevice with D_(total)>D_(m), comprising the fiber and at least one modeconverter that converts the incoming light into the desired HOM of thefiber, such that light propagation occurs substantially in the desiredmode. In some cases, this device will also comprise a mode converter atthe output of the device, so as to obtain a familiar Gaussian spatialpattern of light out of the device.

In one embodiment, the mode converter is a static or tunable long-periodfiber grating.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the index profile of a fiber that supports more than onemode.

FIG. 2 shows the dispersion as a function of wavelength, of the LP₀₂mode, 20, for the fiber described in FIG. 1.

FIG. 3 shows the index profile of a fiber that supports more than onemode, but which is designed to yield desired D_(total) for the LP₀₂mode, while additionally ensuring that the optical signal is stable withrespect to mode coupling.

FIG. 4 shows the dispersion D_(total), for the LP₀₂ mode of thewaveguide described in FIG. 3.

FIG. 5 shows the effective indices n_(eff), of two different modes forthe fiber shown in FIG. 3.

FIG. 6 shows the refractive index profile of a fabricated fiber performthat yields an LP02 mode.

FIG. 7 shows the dispersion of the LP02 mode (Dtotal) for the fiberrepresented in FIG. 6.

FIG. 8 shows the neff of the LP02 mode (80) and LP11 mode (81),respectively.

FIGS. 9( a)(b)(c) show several schematics of exemplary devices.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows the index profile of a fiber that supports more than onemode, but which is designed to yield desired the D_(total) for the LP₀₂mode. The refractive index profile comprises a core, 10, with ΔN of0.039 extending to a radial position of 1 μm; followed by a trenchregion (down-doped ring), 11, with ΔN of −0.008 and a thickness of 0.5μm; followed by an up-doped ring, 12, with ΔN of 0.027 and a thicknessof 1.4 μm. Thereafter, the fiber cladding, consisting only of silicaglass, 13, extends to the edge of the glass cladding of the fiber. Fortypical fibers, this extends to a radial position of 62.5 μm. Theprofile in FIG. 1 is shown only till a radial position of 7 μm becausethe rest of the fiber is merely an extension of the silica glasscladding. The refractive index profile is characterized in terms of ΔN,the difference in refractive index between the region of interest andthe silica cladding.

FIG. 2 shows the dispersion as a function of wavelength, of the LP₀₂mode, 20, for the fiber described in FIG. 1. Also shown is the materialdispersion of silica (21), which is highly negative in the wavelengthrange of 820-900 nm. As a reference, FIG. 2 also shows a dashed line(22), which denotes the zero dispersion line. From this Figure, it isapparent that the LP₀₂ mode of this fiber has large waveguide dispersionD_(w). This is because the dispersion of this mode (D_(total)), variesbetween zero and approximately +5 ps/nm-km, whereas the materialdispersion 13, for the medium varies between −78 ps/nm-km and −109ps/nm-km, in this wavelength range. Since D_(total)=D_(m)+D_(w), andD_(m) is highly negative, D_(w) must be large and positive in order toyields D_(total) that is small and positive.

Those skilled in the art will realize that the waveguide dispersion of amedium is approximately constant with respect to a complimentary scalingof waveguide dimensions and wavelength of operation, as described indetail by Snyder and Love in Optical Waveguide Theory, Chapman and Hall,New York, 1983. The complimentary scaling concept states that awaveguide has approximately the same waveguide dispersion D_(w), for agiven ratio of the dimensional scale of the waveguide to the wavelengthof operation. Hence, those skilled in the art will realise that whileFIG. 1 describes a fiber that yields small positive dispersion in the800-900 nm wavelength range, appropriately scaling the radial dimensionsof the refractive index profile will yield a similar waveguide thatyields the same magnitude of D_(w), for the LP₀₂ mode at an alternativewavelength. Thus, this design template may be used to obtain smallpositive dispersion in any desired wavelength range<1300 nm.

Furthermore, FIG. 2 shows that the dispersion slope, as defined by thederivative of D_(total), with respect to wavelength, is zero, negative,or positive, depending on the wavelength of operation. Hence, with aidof the complimentary scaling concept, it is evident that this designclass can yield dispersion slope of any sign (including zero), whilesimultaneously yielding small, positive D_(total) for wavelengthranges<1300 nm.

FIG. 3 shows the index profile of a fiber that supports more than onemode, but which is designed to yield desired D_(total) for the LP₀₂mode, while additionally ensuring that the optical signal is stable withrespect mode coupling. The refractive index profile comprises a core,30, with ΔN of 0.026 extending to a radial position of 1.1 μm; followedby a trench region, 31, with ΔN of −0.0087 and a thickness of 1.4 μm;followed by an up-doped ring, 32, with ΔN of 0.022 and a thickness of0.7 μm; followed by a trench region, 33, with ΔN of −0.0085 and athickness of 0.7 μm; followed by an up-doped ring, 34, with ΔN of 0.015and a thickness of 1.44 μm; followed by a trench region, 35, with ΔN of−0.0073 and a thickness of 1 μm. Thereafter, the fiber cladding, 36,consisting only of silica glass, extends to the edge of the glasscladding of the fiber.

FIG. 4 shows the dispersion D_(total) for the LP₀₂ mode of the waveguidedescribed in FIG. 3. As is evident, the LP₀₂ mode has large positivedispersion, up to a magnitude of +100 ps/nm-km, at the wavelength of1040 nm. In addition, it has positive dispersion in a wavelength rangespanning 120 nm, from 1040 nm to 1160 nm.

FIG. 5 show the effective indices n_(eff), of two different modes forthe fiber shown in FIG. 3. Line 50 is the n_(eff) of the LP₀₂ mode,which has the large positive D_(total) and which is the desired mode ofoperation, and line 51 is the n_(eff) for the LP₁₁ mode, which hasn_(eff) values closest to those of the desired LP₀₂ mode of this fiber.As is evident from this fiber, the difference in n_(eff) between the twomodes is greater than 10⁻⁴ over the entire preferred wavelength range ofoperation, spanning from 1040 nm to 1160 nm. This large separationbetween the LP₀₂ mode and any other guided mode of this fiber ensuresthat light propagating in the LP₀₂ mode will not easily couple to anyother mode of this fiber, hence ensuring stable, mode-coupling-free,low-loss propagation of the optical signal. The n_(eff) of other guidedmodes in this fiber were not plotted in FIG. 5, because their separationfrom the LP₀₂ mode is even larger, and hence do not contribute toinstabilities due to mode coupling. In general, for this class of fiberdesigns, it is usually necessary to calculate the n_(eff) of everyguided mode, and ensure that the smallest difference in n_(eff) betweenthe desired HOM and any other mode exceeds 10⁻⁴.

Described thus far are two exemplary fiber refractive index profiles.One yields D_(total) greater than the material dispersion D_(m), of themedium (usually silica, with trace dopants), but D_(total)<50 ps/nm-km,in the wavelength range<1300 nm. The other provides for dispersionD_(total)>material dispersion D_(m), with arbitrarily high positivevalues for D_(total) at wavelengths<1300 nm, such that Δn_(eff) betweenthe desired and parasitic modes is always>10⁻⁴, so as so ensure stableoperation. FIG. 6 shows the refractive index profile of a fabricatedfiber preform that yields an LP₀₂ mode which satisfies both the aboveconditions. FIG. 7 shows the dispersion of the LP₀₂ mode (D_(total)) forthe fiber represented in FIG. 6. The LP₀₂ mode has dispersionD_(total)<+40 ps/nm-km, for the wavelength range spanning 100 nm from820-920 nm. Hence, this yields a fiber with low positive dispersion in awavelength range where the material dispersion is large and negative(approximately −100 ps/nm-km, as shown in FIG. 2). Note that thedispersion curve shown in FIG. 7 (as well as the curve shown in FIG. 2)has a turnover region in the wavelength range of interest, i.e. at theoperating wavelength.

FIG. 8 shows the n_(eff) of the LP₀₂ mode (80) and LP₁₁ mode (81),respectively. As in the case of the fiber shown in FIGS. 3, 4, and 5,the LP₁₁ mode has n_(eff) closest to the LP₀₂ mode of this fiber, andhence it suffices to study their difference to evaluate the resistanceto mode coupling here. From FIG. 8, it can be inferred that thedifference in n_(eff) (Δn_(eff)) is greater than 10⁻⁴ over the entiredesired wavelength of operation, spanning from 820-920 nm.

All the discussion above described fibers in which the preferred mode ofoperation was the LP₀₂ mode. The same design techniques may beimplemented to yield a fiber in which propagation occurs in anotherhigher order mode, possessing the dispersive and stabilitycharacteristics described above.

Optical devices using the aforementioned inventive fibers will requiremode converters in order to introduce the optical signal into thepreferred mode of the HOM fiber. Hence an incoming optical beam, usuallyGaussian in spatial pattern because that is the mode of choice forconventional fibers as well as free-space beams, must be spatiallyconverted into the preferred HOM with high efficiency. This can bereadily achieved with suitably designed long-period fiber-gratings,whose operation as both static and dynamic mode converters is describedin detail in U.S. Pat. Nos. 6,084,996 and 6,768,835. Fiber-gratingmode-converters can achieve losses as low as 0.1 dB while providing modeconversion efficiencies as high as 99.99%, as experimentallydemonstrated by Ramachandran et al, and described in Optics Letters,vol. 27, p. 698, 2002, entitled “Bandwidth control of long-periodgrating-based mode converters in few-mode fibers.” An exemplary deviceconstruction using fiber gratings is shown in FIG. 9( a), where the HOMfiber is connected to gratings at the input and output of the fiber, soas to ensure that the input as well as output of the device is aconventional Gaussian mode even though the preferred mode of operationinside the device may be different. The light source and optical path,represented by the arrows, may have any suitable wavelength but ispreferably below 1300 nm. Also shown in this schematic are themodal-images of the conventional lower order mode (LOM), typically LP₀₁,and a desired HOM, preferably LP₀₂, that has the desired dispersive andstability properties. Constructed thus, the device can be concatenatedto any other system, be it an Yb-doped fiber laser that requirespositive dispersion inside the laser cavity, or a pulse delivery schemefor a Ti:Sapphire laser, where the device is used after light exits thesolid-state laser (lasers and systems architectures not shown inFigure). Alternatively, discrete phase plates, as described by Ishaayaet al in Optics Letters, vol. 30, p. 1770, 2005, entitled “Intracavitycoherent addition of single high-order modes.” This schematic is shownin FIG. 9( b). Several applications desire high power beam output thatcan be collimated, and not that they be Gaussian in shape. For suchapplications, the schematic of FIG. 9( c) may be suitable, where anappropriate mode-converter transforms the incoming Gaussian mode intothe desired HOM, but the output is simply collimated and relayed infree-space.

As described, the optical fiber device is designed for propagation of ahigher order mode (HOM) in a section of optical waveguide, preferably anoptical fiber, such that the total dispersion in the section of opticalfiber is greater than the material dispersion in the section of opticalwaveguide. To achieve this, the optical waveguide/fiber supportspropagation of the HOM as the main propagating mode, i.e. with most ofthe optical energy in the preferred HOM. As stated earlier, to ensurethat most of the optical energy remains in the preferred HOM, i.e. isnot mode converted, the preferred HOM has an effective refractive indexthat is different from the effective refractive index for any other modeby at least 0.0001. For the purpose of defining the invention, anoptical fiber having this characteristic may be referred to as a fewmode fiber, i.e., one that supports at least one mode in addition to afundamental mode.

Various additional modifications of this invention will occur to thoseskilled in the art. All deviations from the specific teachings of thisspecification that basically rely on the principles and theirequivalents through which the art has been advanced are properlyconsidered within the scope of the invention as described and claimed.

The present invention is directed to optical fiber devices incorporatingoptical fibers with refractive index profiles that yield Dwal>Dm in anywavelength range such that Dtotal<+50 ps/nm-km, as would be desired in avariety of fiber devices exploiting optical nonlinearities. Theaforementioned refractive index profile yields the given dispersionvalues when light resides substantially in a single higher order mode(HOM) of the fiber. Typically, this HOM would be the LP₀₂ mode of thefiber, but those skilled in the art will realize that such designs canbe extended to other HOMs, such as the LP₁₁ or the LP₀₃ modes. We showexemplary profiles that provide small positive D_(total) (<+50 ps/nm-km)in the wavelength range of 820-900 nm, as well as 1040-1160 nm, sincethese wavelength ranges are especially of interest for nonlinearfiber-optical devices, and it is in these wavelength ranges thatconventional silica fiber has D_(total)<0, motivating the need foralternatives.

1. An all-solid glass optical fiber, having: a material dispersion; anda refractive index profile configured to support a higher-order mode(HOM), the HOM having a total dispersion that is greater than thematerial dispersion at a wavelength that is less than approximately 1300nm, the HOM having a first effective refractive index, the refractiveindex profile further configured to support a second mode, the secondmode having a second effective refractive index, the second effectiverefractive index being different from the first effective refractiveindex by at least 10⁻⁴, the all-solid glass optical fiber beingconfigured to generate broadband supercontinua during propagation of anHOM signal.
 2. The optical fiber of claim 1, the second mode being afundamental mode.
 3. The optical fiber of claim 1, the second mode beinga second HOM.
 4. The optical fiber of claim 1, further being configuredto generate derivative effects of broadband supercontinua duringpropagation of the HOM signal.
 5. The optical fiber of claim 1, furtherbeing configured to generate solitons during propagation of the HOMsignal.
 6. The optical fiber of claim 1, the broadband supercontinuabeing generated when the HOM signal wavelength is between approximately820 nm and approximately 900 nm.
 7. The optical fiber of claim 1, thebroadband supercontinua being generated when the HOM signal wavelengthis between approximately 1040 nm and approximately 1160 nm.
 8. Anall-solid glass optical fiber, having: a material dispersion; and arefractive index profile configured to support a first higher-order mode(HOM), the first HOM having a total dispersion that is greater than thematerial dispersion at a wavelength that is less than approximately 1300nm, the first HOM having a first effective refractive index, therefractive index profile further configured to support other modes, eachof the other modes having its respective effective refractive index,each of the other effective refractive indices being different from thefirst effective refractive index by at least 10⁻⁴, the all-solid glassoptical fiber being configured to generate broadband supercontinuaduring propagation of an HOM signal.
 9. The optical fiber of claim 8,the other modes comprising a fundamental mode.
 10. The optical fiber ofclaim 8, the other modes comprising a second HOM.